Display devices are indispensable necessities for people in daily life. No matter the display devices installed on mobile phones or the display devices for displaying television programs, they are a part of people's daily lives. Due to their great popularity and importance, users regard the quality of display devices as an important issue gradually. The luminance of frames displayed by display devices is one of the key factors for enhancing display qualities. Thereby, luminance adjustment becomes an important issue in research and development for display manufacturers. In the past, adjustment of the luminance for display devices usually needs a complex process in order to get a better luminance. The conventional adjustment technique utilizes a plurality of linear oblique lines to form a line with many connecting points for representing the corresponding relation between an input luminance and an output luminance in rectangular coordinate. Next, the conventional adjustment technique adjusts luminance of image signal according to the line. Due to the process mentioned above, the boundary effect will be occurred. In addition, it needs more process at the connecting points of the plurality of linear oblique lines in the line. That is why the conventional adjustment technique is so complex that it has the slow process and serious distortion. Thereby, the picture quality presented by the display device is reduced.